The development of non-heme N4-tetradentate Manganese(II) complexes for alcohol oxidation

Abstract

A series of known and novel non-heme N₄-tetradentate ligands, R,R- and S,S-L5 - L12, of the BPMCN type (BPMCN = N,N'-dimethyl-N,N'-bis(2-pyridylmethyl)-(R,R/S,S)-1,2-diaminocyclo-hexane) containing different N-donors (pyridine or 1-methylimidazole), substituents on the N-donor (methyl or bromo), degrees of amination (secondary or tertiary) and enantiomers (R,R and S,S), were prepared via an indirect reductive amination and methylation procedure. The ligands were characterised by Fourier transform infrared (FT-IR) and nuclear magnetic resonance (NMR) spectroscopy. The N4-tetradentate ligands were reacted with Mn(OTf)2 to produce the non-heme Mn(II)-complexes, R,R- and S,S-C1 - C8, with general formula [(L)Mn(OTf)₂]. The prepared Mn(II)-complexes were characterised by magnetic susceptibility, melting point analysis, atmospheric pressure chemical ionisation mass spectrometry (APCI-MS), ultraviolet-visible (UV-visible) spectroscopy, single crystal x-ray diffraction (SC-XRD) and CHNS elemental analysis, which confirmed the molecular structure and bulk purity of the complexes. Screening of the complexes against benzyl alcohol oxidation revealed that the tertiary diamine complexes, R,R- and S,S-C5 - C8, were more stable than their secondary diamine counterparts, R,R- and S,S-C1 - C4. Complexes containing the BMIMCN ligand system, R,R- and S,S-C8, showed the highest activity due to the ligands' higher electron donating abilities. Complexes with a ligand system containing a substituent in the C6 position, R,R- and S,S-C2, C3, C6 and C7 displayed lower catalytic activity due to increased steric constraints in the coordination sphere of the metal. No difference in activity was found between the R,R- and S,S-configurations of the Mn(II)-complexes, which confirmed the identical active site accessibility as established by solid-state structural analysis. The best performing complex, S,S-C8, was used to optimise various reaction parameters, i.e. catalyst, H₂O₂ and AcOH concentration. It was found that an increase in all of the parameters resulted in an increase in substrate conversion; however, selectivity towards the aldehyde product decreased. This was due to over-oxidation resulting in the formation of benzoic acid in the product mixture. The optimal conditions from the different parameters were consequently used to oxidise a variety of linear, cyclic, bicyclic and benzylic alcohols. Although primary alcohol substrates could be oxidised with high conversions (97%), they suffered from reduced selectivity and side-reactions. It was decided to extend the study towards secondary alcohol substrates and excellent conversions of up to 100% with isolated yields of up to 86% for the ketone products were achieved. In addition, no unwanted side-reactions or selectivity issues were observed. These results indicated that the novel non-heme N4-tetradentate Mn(II)-complexes, R,R- and S,S-C8, showed high catalytic activity towards a variety of alcohol substrates and, in some cases, proved to be superior to previous Mn(II)-complex systems utilised in catalytic alcohol oxidation.